Blending of materials (liquids or solid particles) usually relies on mechanical means of moving one portion of the material with respect to another portion thus distributing streams of solids with respect to each other. The better mixers will frequently change relative movement direction to produce a crosswise reverse motion of the material. Usually mechanical impellers of various shapes are used, including mechanically activated ribbons and paddles. In some blenders, a series of stationary paddles are used and the material is allowed to drop through the paddles and thus produce a sequence of cuts and deflections of the stream in various directions to produce a mixing action. Sometimes the mechanical impellers are moved fast enough to throw the material. While this sometimes improves mixing, it often degrades the material and consequently does not produce a satisfactory mixing process.
The blender of the present invention has a particular shape defined by the following features. At each elevation above the discharge opening, the cross section of the blender in any plane perpendicular to the axis of symmetry of the blender is racetrack-shaped; that is, the cross section consists of two opposed semicircles, spaced, and with their concave sides facing each other, the ends of the semicircles joined by parallel straight lines, resulting in a shape resembling that of a racetrack. The resulting blender necessarily has an axis of symmetry.
If the diameters of the semicircles are the same at all elevations, then the flat surfaces generated by the parallel straight lines will be vertical. On the other hand, if the diameters of the semicircles increase with increasing elevation, then the flat surfaces generated by the parallel straight lines converge downwardly. These two cases are illustrated, respectively, by the lower and the upper portions of the blender shown in FIGS. 1A through 1C. In both cases, the resulting structure is said to have one-dimensional convergence. In some embodiments described below, more than one blender module of this basic shape are combined in cascade, as shown in FIGS. 3A and 3B.
With the present invention, materials are mixed as they flow by gravity through a blending vessel of racetrack configuration and strike its multiple surfaces. The multiple surfaces of the blending vessel walls cause the material to disperse as it strikes the straight part of the racetrack. The curved portions of the racetrack then force this dispersed material back together, thus causing blending. The blending is enhanced when the blending vessel is designed to cause convergence of the material in only one direction at a time. Generally these directions are perpendicular to each other so that dispersion and mixing occur first in one direction and then in a direction perpendicular to the first. This one-dimensional convergence is not only useful to enhance blending, but also can produce bottom to top sequential discharge of material leaving the blending vessel.
The means for introducing material into the racetrack configuration blending vessel can be as simple as a single chute, or multiple feeders feeding multiple chutes.
In a simple, non-rotating embodiment, multiple blending opportunities are provided by stacking blending vessels and allowing material to fall by gravity from one vessel into the next, as in FIGS. 3A and 3B.
In another embodiment, shown in FIGS. 4A through 4C, a large closed introduction chamber affixed to the top of the blending vessel is alternately filled and emptied by gravity as the blending vessel and chamber are rotated as a unit about a horizontal axis. This configuration, in which the ends of both the blending vessel and the chamber are capped so as to contain the material, allows for the repeated entry of the same material into the same blending vessel as the assembly is rotated about a horizontal axis.
The multiple blending opportunities off the rotated embodiment are enhanced when the introduction chamber has the same size and shape as the blending vessel and is mounted in an inverted posture into the upper end off the blending vessel, as shown in FIGS. 5A through 5B. This provides a mixing opportunity with each half revolution. Blending in this dual racetrack-shaped blender configuration is further enhanced by a go-degree rotation of the racetrack axis of one blending vessel with respect to the other.